/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package com.antgroup.geaflow.state.iterator;

import com.antgroup.geaflow.common.iterator.CloseableIterator;

/**
 * This class is a wrapper iterator, allowing multiple hasNext call but one next call.
 */
public class StandardIterator<T> extends BaseCloseableIterator<T, T> {

    private boolean nextCalled;
    private boolean hasNextValue;
    private T nextValue;

    public StandardIterator(CloseableIterator<T> iterator) {
        super(iterator);
        innerNext();
    }

    private void innerNext() {
        this.hasNextValue = this.iterator.hasNext();
        this.nextValue = this.hasNextValue ? this.iterator.next() : null;
        this.nextCalled = false;
    }

    @Override
    public boolean hasNext() {
        // only next has called, we trigger hasNext method.
        if (nextCalled) {
            innerNext();
        }
        return hasNextValue;
    }

    @Override
    public T next() {
        if (nextValue != null) {
            nextCalled = true;
            T next = nextValue;
            nextValue = null;
            return next;
        }
        return null;
        // throw new NoSuchElementException("hasNext not called or has no next data");
    }
}
